Add live CEK stepper island — interactive stepping debugger

A defisland that lets users type an SX expression, step through CEK
evaluation one transition at a time, and see C/E/K registers update
live. Demonstrates that cek-step is pure data->data.

- cek.sx geography: add ~geography/cek/demo-stepper island with
  source input, step/run/reset buttons, state display, step history
- platform_js.py: register CEK stepping primitives (make-cek-state,
  cek-step, cek-terminal?, cek-value, make-env, sx-serialize) so
  island code can access them

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/sx/ref/platform_js.py

@@ -1508,6 +1508,7 @@ CEK_FIXUPS_JS = '''
     while (!cekTerminal_p(state)) { state = cekStep(state); }
     return cekValue(state);
   };
+
 '''
 
 
@@ -3033,6 +3034,8 @@ def fixups_js(has_html, has_sx, has_dom, has_signals=False, has_deps=False, has_
   if (typeof domOuterHtml === "function") PRIMITIVES["dom-outer-html"] = domOuterHtml;
   if (typeof domInnerHtml === "function") PRIMITIVES["dom-inner-html"] = domInnerHtml;
   if (typeof domTextContent === "function") PRIMITIVES["dom-text-content"] = domTextContent;
+  if (typeof domCreateElement === "function") PRIMITIVES["dom-create-element"] = domCreateElement;
+  if (typeof domAppend === "function") PRIMITIVES["dom-append"] = domAppend;
   if (typeof jsonParse === "function") PRIMITIVES["json-parse"] = jsonParse;
   if (typeof nowMs === "function") PRIMITIVES["now-ms"] = nowMs;
   PRIMITIVES["sx-parse"] = sxParse;